In manufacture of a silicon single crystal according to the CZ method in the prior art, a seed crystal of silicon single crystal is brought into contact with a silicon melt, and then slowly pulled up while being rotated to grow a single crystal ingot. At this time, after the seed crystal is brought into contact with the silicon melt, in order to annihilate dislocations propagated from slip dislocations that are generated in the seed crystal in high density due to thermal shock, so-called a necking process for reducing a crystal diameter to approximately 3 mm to form a neck portion is carried out, then the crystal diameter is increased to a desired diameter to pull a dislocation-free silicon single crystal. Such a necking process is widely known as a Dash Necking method, which is common sense in the case of pulling a silicon single crystal ingot according to the CZ method.
That is, a seed crystal used in the prior art has a shape obtained by providing a notch portion, which is used for setting on a seed holder, in a cylindrical or prismatic single crystal having, e.g., a diameter or each side of approximately 8 to 20 mm, and a lower end shape that is brought into contact with a silicon melt first is a flat surface. Further, to safely pull while bearing with a weight of a single-crystal ingot having a heavy weight, it is hard to reduce a thickness of the seed crystal beyond the above-described value when strength of a material is taken into consideration.
Since the seed crystal having such a shape has a high heat capacity at an end thereof that comes into contact with the melt, an exponential temperature difference is produced in the crystal at the moment that the seed crystal comes into contact with the melt, and slip dislocations are provided in high density. Therefore, to annihilate the dislocations and grow the single crystal, the necking is required.
However, in such a state, even if various necking conditions are selected, a minimum diameter must be narrowed to 4 to 6 mm in order to eliminate dislocations, strength is not sufficient to support the single-crystal ingot with a heavy weight with a recent increase in diameter of the silicon single crystal, and a serious accident such as fracture of this narrow necking portion and falling the single crystal ingot may possibly occur during pulling the single crystal ingot.
To solve such a problem, the inventions like those described in Patent Literature 1 or Patent Literature 2 have been suggested. These inventions improve strength of the necking portion by forming a shape of an end portion of the seed crystal into a shape having a wedge shape or a hollow portion, reducing slip dislocations that are formed when the seed crystal comes into contact with the silicon melt as much as possible so that dislocations can be eliminated even if a diameter of the neck portion is relatively increased.
According to this method, since the thickness of the neck portion can be increased, the strength of the neck portion can be improved to some extent, but the neck portion with slip dislocations is still formed by performing the necking, the strength may be insufficient in case of pulling a single-crystal ingot that has a diameter and a length greatly increased in recent years and also has a weight of 150 Kg or more, and a solution to the root of the problem is yet to be found.
Thus, a method for single-crystallizing a crystal without forming a neck portion based on necking, which leads to the most serious problem in terms of strength, has been developed and suggested (Patent Literature 3). According to this method, as shown in FIG. 2, a seed crystal has a shape that is sharp at an end portion which is brought into contact with a silicon melt or a shape obtained by cutting off a sharp end, the end of the seed crystal is gently brought into contact with the silicon melt (FIG. 2 at (1)), the seed crystal is moved down at a low rate (Vdown) and melted until the end portion of the seed crystal has a desired thickness D (FIG. 2 at (2)), then the seed crystal is slowly moved up (Vup), and a silicon single crystal ingot having a desired diameter is grown without performing necking (FIG. 2 at (3)).
According to this method, when the end of the seed crystal is brought into contact with the silicon melt first, since a contact area is small and a heat capacity of the end portion is small, thermal shock or a sharp temperature gradient is not produced in the seed crystal, and hence the slip dislocation is not introduced. Further, when the seed crystal is then moved down at a low rate and melted until the end portion of the seed crystal has a desired thickness, the rapid temperature gradient is not produced, and hence the slip dislocation is not introduced into the seed crystal even during the melting. Furthermore, when the seed crystal is slowly moved up at last, since the seed crystal has a desired thickness and no dislocation, the necking does not have to be carried out, the strength is sufficient, and the seed crystal is thickened until a desired diameter is obtained, thereby growing the silicon single crystal ingot.
As described above, in a regular necking seeding method, a shape or a method for reducing thermal shock at the time of thermal insulation or heating of the seed crystal above the melt or seeding has been disclosed as a method for reducing initial dislocation density, but there is a limit in thickness of a neck, and it is impossible to follow a single crystal ingot having an increased diameter and an increased weight. Thus, a dislocation-free seeding method that can bear with an increase in diameter and an increase in weight mentioned above without performing the necking has been established.
A problem of this dislocation-free seeding method is a success rate for dislocation-free crystal. That is, in this method, when a dislocation is once introduced into the seed crystal, a retry process cannot be effected unless the seed crystal is replaced, and hence improving the success rate is particularly important. Moreover, in this case, even if the seeding is performed without dislocation, the slip dislocation is produced when a tapered end portion of the seed crystal is left in the vicinity of a silicon melting point after melting a predetermined length, or depending on a time required to start crystal growth or a growth rate at the time of shifting to growth of the single crystal, thus resulting in a problem that this dislocation increases.
Thus, to enhance the success rate in the dislocation-free seeding method, the inventions like those in Patent Literature 4, Patent Literature 5, and Patent Literature 6 have been suggested. These inventions suggest a retention time before the melting, a temperature at the melting, a melting rate, a retention time after the melting, a growth rate and a magnetic field intensity during crystal growth, and so on.
In the inventions suggested to enhance the success rate in the dislocation-free seeding method, as a common important item, there are contents that a silicon melt surface temperature T at which the seed crystal is melted is set to be higher than a melting point of silicon. According to Patent Literature 4, there is a description about the range that is higher than the silicon melting point by 25° C. or more and 45° C. or less. According to Patent Literature 5, there is a description that melting is effected at a temperature that a diameter of the crystal is reduced to be smaller than a diameter after end of melting by 0.3 mm or more and 2 mm or less in a section that is 3 mm for pulling from a position of end of the melting of the seed crystal. According to Patent Literature 6, there is a description about a temperature that is 10 to 20° C. higher than a temperature that is a suitable temperature in the Dash Necking method.
The necessity of melting the seed crystal at a temperature higher than the silicon melting point is that melting at the high temperature enables completely melting the seed crystal without leaving an unmelted part in the end portion of the seed crystal. That is because, in a state that a temperature of the silicon melt is not sufficiently high, the seed crystal in a solid state sinks into the silicon melt without rapidly melting the end of the seed crystal and the slip dislocation is produced.
Additionally, in an increase in weight due to recent realization of a long size of a pulled crystal or in an ultra-heavy-weight crystal like a next-generation crystal having a diameter of 450 mm, a minimum diameter required for supporting a single crystal ingot increases. In the next-generation crystal having a diameter of 450 mm in particular, a crystal weight exceeding one ton is assumed, in this case a minimum diameter of 10 mm or more is required for holding. When the minimum diameter of 10 mm is required, the seed crystal needs to be melted up to a portion having a diameter of 10 mm or more. When this diameter at the melting increases, even a thicker portion needs be rapidly melted, and hence a higher temperature is needed to melt this portion.
Unfortunately, the increase in the temperature at the melting required for increasing the success rate in the dislocation-free seeding method causes the following problem.
The problem of the increase in the temperature at the melting appears when the process shifts to the crystal growth after end of melting the seed crystal. The melting process smoothly advances if a temperature of the silicon melt is set to be higher than the melting point. However, if a temperature of the silicon melt immediately after end of melting is higher than the melting point, a diameter of the crystal is constricted to be smaller than a diameter immediately after end of melting as a matter of course when the crystal growth begins as it is, and the crystal is cut in some cases. Even if the crystal is not cut, the narrowed diameter results in a problem that strength is insufficient for holding the heavy-weight crystal.
Further, according to Patent Literature 4, there is a description that shifting to the crystal growth during a period of 0 to 10 minutes after end of melting enables improving the dislocation-free success rate but the dislocation-free success rate is lowered if this time becomes long. In case of shifting to the crystal growth during the period of 0 to 10 minutes after end of melting when the temperature at the melting is high, there is a problem that a temperature of the silicon melt cannot be rapidly lowered, a diameter is reduced after start of crystal growth, and strength is insufficient for holding the heavy-weight crystal.
An amount of silicon melt in a crucible in which a heavy-weight crystal is manufactured has a heavy weight and a high heat capacity, and a problem is to reduce a temperature of the silicon melt in a short time. To reduce the temperature of the silicon melt, a technique for lowering electric power of a heating graphite heater installed in a furnace is used, but a change in silicon melt temperature corresponding to a change in electric power of the heating graphite heater has a poor response, and hence it is very difficult to rapidly reduce the temperature of the silicon melt within a short time immediately after end of melting the seed crystal to start of the crystal growth.
In the conventional methods, the temperature at the melting is accordingly limited to ensure that the crystal diameter at the beginning of crystal growth after the melting is large enough to hold a heavy crystal body, although the temperature is set to be somewhat high. The above limitation hinders the temperature at the melting from being set to be sufficiently high to increase the success rate for dislocation-free crystal, resulting in a lower success rate for dislocation-free crystal.